Interpretive Summary: We examined the ability of 45 isolates of 16 Pythium species found in the soil of forest nurseries for their ability to cause disease to Douglas-fir seedlings. This was examined by planting Douglas-fir seeds into soil infested with each of the Pythium isolates separately. We found that eight of the 16 species significantly reduced seedling survival. These results are important because they help nursery managers evaluate disease risk associated with the presence of particular Pythium species. Pythium species that were not found to cause disease in this study are likely pose less of a disease risk if they are found in a forest nursery. The nursery manager may therefore wish to postpone implementing disease control measures if these nonpathogenic Pythium species are found. However, if pathogenic Pythium species are found in a nursery (as determined by this study) the nursery manager may wish to implement disease control measures to reduce seedling death in the nursery field.

Technical Abstract:
Pythium species are common soilborne oomycetes that occur in forest nursery soils in the Pacific Northwest (PNW) region of the United States. Numerous species have been described. However, with the exception of P. aphanidermatum, P. irregulare, P.mamillatum, and P. ultimum, little is known about the potential for other Pythium species present in nursery soils to cause damping-off of tree seedlings. A potted greenhouse study was conducted to evaluate the pathogenicity of 45 isolates representing 16 Pythium species obtained from the soil at three forest nurseries in Washington (WA) and Oregon (OR). Seeds of Douglas-fir (Pseudotsuga menziesii) were planted into soil infested with each of the isolates. The number of surviving seedlings, the percentage of surviving seedlings with tap root necrosis, and tap root length were evaluated 5 weeks later. Significant variation was observed between isolates and species of Pythium. Eight species (P. dissotocum, P. irregulare, P. aff. macrosporum, P. mamillatum, P. aff. oopapillum, P. rostratifingens, P. sylvaticum, and P. ultimum var. ultimum) signifcantly reduced the number of surviving seedlings compared to seeds planted in noninfested soil (negative control). However, all Pythium species caused significantly greater tap root necrosis (approximately 40%) than was observed on noninoculated seedlings (17%). Tap root length varied little between Pythium species and was not a useful character for evaluating pathogencity.